1. Field of the Invention
The present invention relates to an image pickup apparatus for photoelectrically converting an optical image focused to an image sensor to thereby output a corresponding image signal and an image pickup control method therefor. More particularly, the present invention relates to an image pickup apparatus capable of outputting, when taking a still picture, an image signal while broadening the dynamic range of an image sensor and an image pickup control method therefor.
2. Description of the Background Art
A digital camera extensively used today includes a solid-state image sensor implemented by CCD (Charge Coupled Device) charge transfer paths or MOS (Metal Oxide Semiconductor) devices. In the image sensor, photosensitive cells implemented by photodiodes photoelectrically transform incident light to corresponding signal charges. The signal charges are read out to generate digital image data representative of a scene picked up. To meet an increasing demand for high definition images, digital still cameras with 2,000,000 or more pixels have recently been put on the market. The photosensitive array of the image sensor is not increasing in size with an increase in the number of pixels, but is decreasing in order to, e.g., increase yield. This allows optics including a lens to be reduced in size and therefore contributes to the reduction of the overall size and weight of the camera.
In the image sensor of the kind having a high pixel density arranged therein, the area and capacity to be allocated to the individual photodiode must be reduced. It is therefore necessary to reduce noise ascribable to the image sensor, i.e., to increase the S/N (Signal-to-Noise) ratio and to realize a dynamic range broad enough to sufficiently reproduce low brightness to high brightness. However, a decrease in the area of the individual photodiode directly translates into a decease in a signal charge generated thereby. This, coupled with the fact that a signal charge to be stored in the individual photodiode decreases with a decrease in capacity, makes it difficult to noticeably broaden the dynamic range of the image sensor of the kind described.
A progressive scan, CCD image sensor with a xc2xc 300,000 pixel configuration is disclosed in ITE Technical Report, Vol. 20, No. 23, pp. 43-48, Mar. 19, 1996 as a solution to the above-described problem. In accordance with this document, the maximum saturation charge of the image sensor is increased. Information identical with information stored in IT (Interline Transfer)-CCDs and information derived from a short exposure time are transferred to vertical transfer paths at the same time and then combined together by external circuitry.
Japanese patent laid-open publication No. 141229/1994 teaches an image pickup apparatus constructed to produce a composite image with a broad dynamic range by picking up two or more images different in charge storage time and combining them together. Japanese Patent No. 2755366 proposes an electronic camera capable of combining image data derived from a first exposure time and image data derived from a second exposure time longer than the first exposure time, thereby broadening the dynamic range of an image sensing array. Further, Japanese patent publication No. 97841/1995 discloses an image pickup apparatus constructed to select, out of signal levels of various portions of a plurality of frames that are different in the amount of exposure, portions having adequate levels and combining them together to output a composite image.
In any case, it has been customary to vary an exposure time determined by an electronic shutter function available with CCDs so as to pick up an image two times during a single field period. The resulting two images are combined in order to broaden the dynamic range.
However, a problem with the conventional technologies is that noticeable smears appear in an image signal derived from a high speed shutter although the dynamic range may be broadened. This, coupled with the fact that the image signal derived from a high speed shutter is increased in gain, degrades the combined image. More specifically, assume that while first pixel signals derived from a high speed shutter, which is opened only for a short period of time, are being transferred along vertical transfer paths, a second shot is effected over a period of time longer than the above period of time. Then, signal charges are caused to leak to the vertical transfer paths due to the second exposure and mixed with the first image signal, causing smears to appear in the first image signal. Smears are particularly conspicuous when a great amount of exposure is assigned to the second exposure.
Further, it is difficult to accurately combine two images because image signals are corrected in level in accordance with a ratio between the shutter open times and then combined. For example, to effect such consecutive exposure with a mechanical shutter and accurate shutter speeds, a highly accurate shutter opening and closing mechanism is required and must be provided with a sophisticated configuration for insuring stable operation. Such a shutter mechanism is therefore not feasible for a small size, light weight handy camera.
Moreover, even if exposure conditions for a shot are adequate, a scene to be shot sometimes includes a shadow portion and a highlight portion. In a highlight portion, in particular, the photodiodes of the image sensor are likely to saturate due to the brightness of a subject and the scatter of the image sensor itself. Therefore, simply combining two images is not successful to guarantee an attractive processed image.
It is therefore an object of the present invention to provide an image pickup apparatus capable of picking up a scene with a minimum of influence of smears and thereby producing an image signal with an adequately broadened dynamic range, and an image pickup control method therefor.
An image pickup apparatus of the present invention includes a shutter for intercepting, when an exposure time expires with photosensitive cells included in an image sensor generating signal charges, light incident to the image sensor. Image signals output from the image sensor are written to a memory. A controller determines an amount of exposure for picking up a scene in the form of a still picture. A combining circuit combines the image signals read out of the memory. The controller includes an exposure value determining circuit for determining a first exposure value based on the incident light and a second exposure value that effects lower exposure than the first exposure value. An exposure control circuit causes the optical image to be photoelectrically converted over the exposure time that consists of a first and a second period of time corresponding to the first and second exposure values, respectively. A first read control circuit causes, at a timing corresponding to the first period of time, signal charges generated by the photosensitive cells to be shifted to the transfer paths of the image sensor, thereby causing the image sensor to output a first signal corresponding to the signal charges. A light intercepting circuit causes, at the end of the exposure time, the shutter to intercept the incident light to thereby end the second period of time following the first period of time. A second read control circuit causes, after the image sensor has output the first signal, the image sensor to output a second signal corresponding to signal charges generated by the photosensitive cells during the second period of time. A ratio calculating circuit calculates an exposure amount ratio between the first and second signals. A correcting circuit corrects the level of the second signal in accordance with the exposure amount ratio. The combining circuit combines image signals, which respectively correspond to the first signal and the second signal corrected by the correcting circuit, for thereby outputting an image signal representative of a single frame picked up during the exposure time.
An image pickup control method of the present invention is applicable to the above image forming apparatus.